1. Field of the Invention
This invention relates to a method for deciding a duty factor and a driving method using the duty factor, in a light-emitting device in which a unit for supplying a current to a light-emitting element and a light-emitting element are provided in each of plural pixels. A light-emitting device includes a panel having a light-emitting element sealed therein, and a module in which IC and the like including a controller are mounted on the panel.
2. Description of the Related Art
In an active-matrix light-emitting device, the gradation is controlled by a video signal written into each pixel. Hereinafter, a method for driving a light-emitting device using an analog video signal will be described.
In the following example, a frame frequency of k is used. As shown in FIG. 12, when the frame frequency is k, k frame periods are provided in one second. A frame period is equivalent to a period during which a video signal is written to all the pixels and display of one screen is performed.
In each frame period, as an analog video signal is written to each pixel, the luminance of the light-emitting element of each pixel is controlled in accordance with image information held by the analog video signal, and the gradation is thus displayed. In writing an analog video signal to the pixels, a so-called point-sequential format for sequentially writing to each pixel, or a so-called line-sequential format for sequentially writing to each pixel of each line may be used. In both formats, a period during which an analog video signal is written to all the pixels is equivalent to a writing period Ta.
After the writing of an analog video signal ends, a holding period Ts starts and the luminance of the light-emitting element in each pixel is held until the frame period ends.
When the above-described driving method is used, the pixels perform display both in the writing period Ta and the holding period Ts. Therefore, depending on image information held by an analog video signal, the pixels may be constantly on, that is, the light-emitting elements of the pixels constantly emit light. A period during which actual display is performed is called display period.
In the case of the driving method shown in FIG. 12, the writing period Ta and the holding period Ts altogether form the display period.
Although in FIG. 12, one frame period is divided into the writing period Ta and the holding period Ts, it may be the writing period Ta alone without providing the holding period Ts. In short, immediately after an analog video signal is written to each pixel, the next frame period starts and writing of an analog video signal to each pixel is started again. Also in this case, the pixels may constantly be on, depending on image information held by an analog video signal. Therefore, in the case of this driving method, the writing period Ta itself is equivalent to the display period.
While display is performed both in the writing period Ta and the holding period Ts in FIG. 12, display may be performed only in the holding period Ts without performing display in the writing period Ta. In this case, all the pixels are off, that is, none of the light-emitting elements of the pixels emits light at all in the writing period Ta, irrespective of image information held by an analog video signal. Then, in the holding period, the luminance of the light-emitting elements is controlled in accordance with an analog video signal. Therefore, in the case of this driving method, the holding period Ts alone is equivalent to the display period.
Meanwhile, the problem in practical application of the light-emitting device is the short lifetime of the light-emitting element due to deterioration of its electroluminescence layer. FIG. 13 shows a change in luminance with the lapse of time with respect to a current flowing through the light-emitting element. As shown in FIG. 13, as the electroluminescence material deteriorates with the lapse of time, the luminance of the light-emitting element is lowered with respect to the current flowing through the light-emitting element.
The deterioration of the electroluminescence material is accelerated by moisture, oxygen, light and heat. Specifically, the rate of deterioration is affected by the structure of a device for driving the light-emitting device, the characteristics of the electroluminescence material, the material of electrodes, the conditions of preparation process, the method for driving the light-emitting device and the like.
Particularly, as a greater quantity of current flows through the light-emitting element, the light-emitting element deteriorates more quickly. When the light-emitting element deteriorates, the luminance of the light-emitting element is lowered even if the voltage applied to the electroluminescence layer is constant. As a result, a displayed image is unclear.